Courses

# Self Ionization of Water and Buffer Class 11 Notes | EduRev

## Class 11 : Self Ionization of Water and Buffer Class 11 Notes | EduRev

``` Page 1

Self ionization of water
Concentration of H
2
O ([H
2
O]) =
1000 / 18
55.5 M
1
?
Water is very weak electrolyte
H
2
O      H
+
+   OH
–
55.5
55.5 – 10
–7
10
–7
10
–7
(at 25
o
C)
?
O] [H
] [OH ] [H
k
2
–
eq
?
?
Constant
? K
w
= k
eq
[H
2
O]  = [H
+
] [OH
–
]
? K
w
= [H
+
]  [OH
–
],at  25
o
C, [H
+
] = [OH
–
] = 10
–7
K
w
(25
o
C)    = 10
–14

?
Self ionization constant of water
let,
1
w
K
at temperature  T
1
,
2
w
K
at temperature  T
2
?
?
?
?
?
?
? ?
?
2 1 w
w
T
1
–
T
1

R 2.303
H

K
K
log
1
2
, ? ? ? K      T
w
? For calculation of ? :
H
2
O      H
+
+  OH
–
c         0 0
c  (1 – ?)       c ? ? ??? ? ? ?? ?c ?
Page 2

Self ionization of water
Concentration of H
2
O ([H
2
O]) =
1000 / 18
55.5 M
1
?
Water is very weak electrolyte
H
2
O      H
+
+   OH
–
55.5
55.5 – 10
–7
10
–7
10
–7
(at 25
o
C)
?
O] [H
] [OH ] [H
k
2
–
eq
?
?
Constant
? K
w
= k
eq
[H
2
O]  = [H
+
] [OH
–
]
? K
w
= [H
+
]  [OH
–
],at  25
o
C, [H
+
] = [OH
–
] = 10
–7
K
w
(25
o
C)    = 10
–14

?
Self ionization constant of water
let,
1
w
K
at temperature  T
1
,
2
w
K
at temperature  T
2
?
?
?
?
?
?
? ?
?
2 1 w
w
T
1
–
T
1

R 2.303
H

K
K
log
1
2
, ? ? ? K      T
w
? For calculation of ? :
H
2
O      H
+
+  OH
–
c         0 0
c  (1 – ?)       c ? ? ??? ? ? ?? ?c ?

K
w
= c ? × c ?
?
c
k

w
? ? c = 5 5.56 M ?
w
–2
K 10  1.8  ? ? ?
?
pH
potent
?
strength
H ydrogen ion
(pH s cale is given by S orension)
? pH     =  – log [H
+
]
p OH =  – log [OH
–
]
p K
w
=  – log K
w

at 25°C
Acidic basic
0
7 14
Neutral
p scale
H
at any temperature :
[H
+
] = [OH
–
]
[H
+
]  [OH
–
] = K
w
? – log [H
+
] – log [OH
–
] = – log [K
w
]
? pH + pOH = p K
w
at 25
o
C at 80
o
C
K
w
= 10
–14
K
w
= 10
–12
pH + pOH = 14 pH + pOH = 12
Some concept regarding pH calculation
? Concept 1 : ? Concept 2:
5 L
pH = 3  ?
1 L
pH = ?
3 L
pH = 3  +
2 L
pH = 3  ?
5 L
pH = ?
? Concept 3: ? Concept 4:
2 L
pH = 3 +
8 L
HO
2
=
10 L
pH = ?
2 L
pH = 13 +
8 L
water =
10 L
pH = ?
? Note :
7
pH
(i) Calculate pOH
(ii) pH = 14- pOH
Calculate  pH
? Concept 5:
pH = 3
+
pH = 4
=
pH = ?
Page 3

Self ionization of water
Concentration of H
2
O ([H
2
O]) =
1000 / 18
55.5 M
1
?
Water is very weak electrolyte
H
2
O      H
+
+   OH
–
55.5
55.5 – 10
–7
10
–7
10
–7
(at 25
o
C)
?
O] [H
] [OH ] [H
k
2
–
eq
?
?
Constant
? K
w
= k
eq
[H
2
O]  = [H
+
] [OH
–
]
? K
w
= [H
+
]  [OH
–
],at  25
o
C, [H
+
] = [OH
–
] = 10
–7
K
w
(25
o
C)    = 10
–14

?
Self ionization constant of water
let,
1
w
K
at temperature  T
1
,
2
w
K
at temperature  T
2
?
?
?
?
?
?
? ?
?
2 1 w
w
T
1
–
T
1

R 2.303
H

K
K
log
1
2
, ? ? ? K      T
w
? For calculation of ? :
H
2
O      H
+
+  OH
–
c         0 0
c  (1 – ?)       c ? ? ??? ? ? ?? ?c ?

K
w
= c ? × c ?
?
c
k

w
? ? c = 5 5.56 M ?
w
–2
K 10  1.8  ? ? ?
?
pH
potent
?
strength
H ydrogen ion
(pH s cale is given by S orension)
? pH     =  – log [H
+
]
p OH =  – log [OH
–
]
p K
w
=  – log K
w

at 25°C
Acidic basic
0
7 14
Neutral
p scale
H
at any temperature :
[H
+
] = [OH
–
]
[H
+
]  [OH
–
] = K
w
? – log [H
+
] – log [OH
–
] = – log [K
w
]
? pH + pOH = p K
w
at 25
o
C at 80
o
C
K
w
= 10
–14
K
w
= 10
–12
pH + pOH = 14 pH + pOH = 12
Some concept regarding pH calculation
? Concept 1 : ? Concept 2:
5 L
pH = 3  ?
1 L
pH = ?
3 L
pH = 3  +
2 L
pH = 3  ?
5 L
pH = ?
? Concept 3: ? Concept 4:
2 L
pH = 3 +
8 L
HO
2
=
10 L
pH = ?
2 L
pH = 13 +
8 L
water =
10 L
pH = ?
? Note :
7
pH
(i) Calculate pOH
(ii) pH = 14- pOH
Calculate  pH
? Concept 5:
pH = 3
+
pH = 4
=
pH = ?

2       :     1
? Note : The final pH of solution after mixing two solution is in between the previous solution pH.
? Concept 6: ? Concept 7:
pH = 3
V
+   pH = 11
V
?   pH = ?
2 V
pH = ?
10 M HCl
–8
? Note :
When [H
+
] > 10
–5
? [H
+
]
water
neglected
When [H
+
] < 10
–5
? [H
+
]
water
considered
H
2
O      H
+
+   OH
–
55.56         10
–8
55.56 – x        10
–8
+ x    x
? x (10
–8
+ x) = 10
–14
?      x = .94 × 10
–7
? [H
+
] = 10
–8
+ .94 × 10
–7
= 1.04 × 10
–7
? pH = 7 – log (1.04)
? Concept 8:
pH = ?
10 M H Cl
2
? Dissociation of weak acid :
H
+
A
–
H
+
+  A
–
c
c (1– ?) c ?      c ?
?
?
?
– 1
c
K
2
a
for weak acid ? ?< < 1 K
a
=  c ?
2
[H
+
] = c ? ? =
a
K c , pH = – log
a
K c ,   for  weak bases    [H
+
] =
b
w
K c
K
?
=
] OH [
K
w
?
? Note :
(a) HA
1
c
K
a
1
||
HA
2
c
K
a
2
?
2
1
a
a
2
1
K
K

] H [
] H [
?
?
?
Page 4

Self ionization of water
Concentration of H
2
O ([H
2
O]) =
1000 / 18
55.5 M
1
?
Water is very weak electrolyte
H
2
O      H
+
+   OH
–
55.5
55.5 – 10
–7
10
–7
10
–7
(at 25
o
C)
?
O] [H
] [OH ] [H
k
2
–
eq
?
?
Constant
? K
w
= k
eq
[H
2
O]  = [H
+
] [OH
–
]
? K
w
= [H
+
]  [OH
–
],at  25
o
C, [H
+
] = [OH
–
] = 10
–7
K
w
(25
o
C)    = 10
–14

?
Self ionization constant of water
let,
1
w
K
at temperature  T
1
,
2
w
K
at temperature  T
2
?
?
?
?
?
?
? ?
?
2 1 w
w
T
1
–
T
1

R 2.303
H

K
K
log
1
2
, ? ? ? K      T
w
? For calculation of ? :
H
2
O      H
+
+  OH
–
c         0 0
c  (1 – ?)       c ? ? ??? ? ? ?? ?c ?

K
w
= c ? × c ?
?
c
k

w
? ? c = 5 5.56 M ?
w
–2
K 10  1.8  ? ? ?
?
pH
potent
?
strength
H ydrogen ion
(pH s cale is given by S orension)
? pH     =  – log [H
+
]
p OH =  – log [OH
–
]
p K
w
=  – log K
w

at 25°C
Acidic basic
0
7 14
Neutral
p scale
H
at any temperature :
[H
+
] = [OH
–
]
[H
+
]  [OH
–
] = K
w
? – log [H
+
] – log [OH
–
] = – log [K
w
]
? pH + pOH = p K
w
at 25
o
C at 80
o
C
K
w
= 10
–14
K
w
= 10
–12
pH + pOH = 14 pH + pOH = 12
Some concept regarding pH calculation
? Concept 1 : ? Concept 2:
5 L
pH = 3  ?
1 L
pH = ?
3 L
pH = 3  +
2 L
pH = 3  ?
5 L
pH = ?
? Concept 3: ? Concept 4:
2 L
pH = 3 +
8 L
HO
2
=
10 L
pH = ?
2 L
pH = 13 +
8 L
water =
10 L
pH = ?
? Note :
7
pH
(i) Calculate pOH
(ii) pH = 14- pOH
Calculate  pH
? Concept 5:
pH = 3
+
pH = 4
=
pH = ?

2       :     1
? Note : The final pH of solution after mixing two solution is in between the previous solution pH.
? Concept 6: ? Concept 7:
pH = 3
V
+   pH = 11
V
?   pH = ?
2 V
pH = ?
10 M HCl
–8
? Note :
When [H
+
] > 10
–5
? [H
+
]
water
neglected
When [H
+
] < 10
–5
? [H
+
]
water
considered
H
2
O      H
+
+   OH
–
55.56         10
–8
55.56 – x        10
–8
+ x    x
? x (10
–8
+ x) = 10
–14
?      x = .94 × 10
–7
? [H
+
] = 10
–8
+ .94 × 10
–7
= 1.04 × 10
–7
? pH = 7 – log (1.04)
? Concept 8:
pH = ?
10 M H Cl
2
? Dissociation of weak acid :
H
+
A
–
H
+
+  A
–
c
c (1– ?) c ?      c ?
?
?
?
– 1
c
K
2
a
for weak acid ? ?< < 1 K
a
=  c ?
2
[H
+
] = c ? ? =
a
K c , pH = – log
a
K c ,   for  weak bases    [H
+
] =
b
w
K c
K
?
=
] OH [
K
w
?
? Note :
(a) HA
1
c
K
a
1
||
HA
2
c
K
a
2
?
2
1
a
a
2
1
K
K

] H [
] H [
?
?
?

? So HA
2
is stronger acid than HA
1
(b) HA
1
c
1
K
a
1
HA
2
c
2
K
a
2
2 2
1 1
2
1
Ka c
ka c

] H [
] H [
?
?
?
? The acidic strength of two weak acid can be compared by K
a
value only in the case when their concen-
tration are same. Otherwise the procedure of case  (b) should be followed .
?
pH  of mixture of two acids
?
? Case (I) (Strong acid + weak acid)
Question : HCl (0.1 M) + CH
3
COOH (0.2 M)

?
K
a
= 10
–5
CH
3
COOH  CH
3
COO
–
+   H
+
t = 0     0.2      0          0. 1
t = t
eq
0.2 – x      x          0.1 + x (x is very small)

?

?

?
0.2       x 0.1
? pH = – log (0.1) = 1 ? [CH
3
COO
–
] = ? ?
5 – –
3
5 –
10  2  ] COO [CH
2 . 0
1 . 0 x
10 ? ? ?
?
?
?    Case (II-A) (weak acid + weak acid ) (k
as
are incomparable)
Question : HA
1
(0.1 M,
K
a
1
= 10
–5
)
HA
2
(0.2 M,
K
a
2
= 10
–9
)
HA
1
H
+
+ A
1
–
HA
2
H
+
+ A
2
–
0.1      0 0 0.2        0 0
0.1 – x     x + y x 0.2 – y        x + y  y       (
K
a
1
>>
K
a
2
?

? ? ?

?

?
? x  > > y)
0.1       x x  0.2           x  y
x and y are also very less
Page 5

Self ionization of water
Concentration of H
2
O ([H
2
O]) =
1000 / 18
55.5 M
1
?
Water is very weak electrolyte
H
2
O      H
+
+   OH
–
55.5
55.5 – 10
–7
10
–7
10
–7
(at 25
o
C)
?
O] [H
] [OH ] [H
k
2
–
eq
?
?
Constant
? K
w
= k
eq
[H
2
O]  = [H
+
] [OH
–
]
? K
w
= [H
+
]  [OH
–
],at  25
o
C, [H
+
] = [OH
–
] = 10
–7
K
w
(25
o
C)    = 10
–14

?
Self ionization constant of water
let,
1
w
K
at temperature  T
1
,
2
w
K
at temperature  T
2
?
?
?
?
?
?
? ?
?
2 1 w
w
T
1
–
T
1

R 2.303
H

K
K
log
1
2
, ? ? ? K      T
w
? For calculation of ? :
H
2
O      H
+
+  OH
–
c         0 0
c  (1 – ?)       c ? ? ??? ? ? ?? ?c ?

K
w
= c ? × c ?
?
c
k

w
? ? c = 5 5.56 M ?
w
–2
K 10  1.8  ? ? ?
?
pH
potent
?
strength
H ydrogen ion
(pH s cale is given by S orension)
? pH     =  – log [H
+
]
p OH =  – log [OH
–
]
p K
w
=  – log K
w

at 25°C
Acidic basic
0
7 14
Neutral
p scale
H
at any temperature :
[H
+
] = [OH
–
]
[H
+
]  [OH
–
] = K
w
? – log [H
+
] – log [OH
–
] = – log [K
w
]
? pH + pOH = p K
w
at 25
o
C at 80
o
C
K
w
= 10
–14
K
w
= 10
–12
pH + pOH = 14 pH + pOH = 12
Some concept regarding pH calculation
? Concept 1 : ? Concept 2:
5 L
pH = 3  ?
1 L
pH = ?
3 L
pH = 3  +
2 L
pH = 3  ?
5 L
pH = ?
? Concept 3: ? Concept 4:
2 L
pH = 3 +
8 L
HO
2
=
10 L
pH = ?
2 L
pH = 13 +
8 L
water =
10 L
pH = ?
? Note :
7
pH
(i) Calculate pOH
(ii) pH = 14- pOH
Calculate  pH
? Concept 5:
pH = 3
+
pH = 4
=
pH = ?

2       :     1
? Note : The final pH of solution after mixing two solution is in between the previous solution pH.
? Concept 6: ? Concept 7:
pH = 3
V
+   pH = 11
V
?   pH = ?
2 V
pH = ?
10 M HCl
–8
? Note :
When [H
+
] > 10
–5
? [H
+
]
water
neglected
When [H
+
] < 10
–5
? [H
+
]
water
considered
H
2
O      H
+
+   OH
–
55.56         10
–8
55.56 – x        10
–8
+ x    x
? x (10
–8
+ x) = 10
–14
?      x = .94 × 10
–7
? [H
+
] = 10
–8
+ .94 × 10
–7
= 1.04 × 10
–7
? pH = 7 – log (1.04)
? Concept 8:
pH = ?
10 M H Cl
2
? Dissociation of weak acid :
H
+
A
–
H
+
+  A
–
c
c (1– ?) c ?      c ?
?
?
?
– 1
c
K
2
a
for weak acid ? ?< < 1 K
a
=  c ?
2
[H
+
] = c ? ? =
a
K c , pH = – log
a
K c ,   for  weak bases    [H
+
] =
b
w
K c
K
?
=
] OH [
K
w
?
? Note :
(a) HA
1
c
K
a
1
||
HA
2
c
K
a
2
?
2
1
a
a
2
1
K
K

] H [
] H [
?
?
?

? So HA
2
is stronger acid than HA
1
(b) HA
1
c
1
K
a
1
HA
2
c
2
K
a
2
2 2
1 1
2
1
Ka c
ka c

] H [
] H [
?
?
?
? The acidic strength of two weak acid can be compared by K
a
value only in the case when their concen-
tration are same. Otherwise the procedure of case  (b) should be followed .
?
pH  of mixture of two acids
?
? Case (I) (Strong acid + weak acid)
Question : HCl (0.1 M) + CH
3
COOH (0.2 M)

?
K
a
= 10
–5
CH
3
COOH  CH
3
COO
–
+   H
+
t = 0     0.2      0          0. 1
t = t
eq
0.2 – x      x          0.1 + x (x is very small)

?

?

?
0.2       x 0.1
? pH = – log (0.1) = 1 ? [CH
3
COO
–
] = ? ?
5 – –
3
5 –
10  2  ] COO [CH
2 . 0
1 . 0 x
10 ? ? ?
?
?
?    Case (II-A) (weak acid + weak acid ) (k
as
are incomparable)
Question : HA
1
(0.1 M,
K
a
1
= 10
–5
)
HA
2
(0.2 M,
K
a
2
= 10
–9
)
HA
1
H
+
+ A
1
–
HA
2
H
+
+ A
2
–
0.1      0 0 0.2        0 0
0.1 – x     x + y x 0.2 – y        x + y  y       (
K
a
1
>>
K
a
2
?

? ? ?

?

?
? x  > > y)
0.1       x x  0.2           x  y
x and y are also very less
Corporate Office: 97/1, 3F, Adhchini, Sri Aurobindo Marg, Near NCERT, New Delhi |  011-32044009
53
Haryana Office : SCO-51, Sector-13, Shopping Complex, Hisar, Haryana | Mob.: 9896810009
1 –
5 –
10
x . x
10 ?
[H
+
] = x = 10
–3
,  ? pH = 3 [A
1
–
] = [A
2
–
] =
? Case (II-B) (weak acid + weak acid ) (k
as
are comparable)
?Question : HA
1
(0.1)
K
a
1
= 10
–5
HA
2
(0.2)
K
a
2
= 10
–6
HA
1
H
+
+ A
1
–
HA
2
H
+
+ A
2
–
0.1      0 0 0.2        0 0
0.1 – x     x + y x 0.2 – y        x + y y
0.1     x + y x 0.2        x + y y
? now
0.1
y) (x x
K
1
a
?
?
?
1
a
c
y) (x x
K
1
?
?
,
2
a
c
y y) (x
K
2
?
?
?
?
2 1
a 2 a 1
K c  K  c y x  ] [H ? ? ? ?
?
? weak polyprotic acids or bases
(H
2
S, H
3
PO
4
, NH
2
– NH
2
, H
2
CO
3
)
(a) H
2
S     H
+
+ HS
–
K
a
1
HS
–
H
+
+ S
2–
K
a
2
(b) NH
2
–  NH
2
+ H
+
NH
2
– NH
3
+
1
b
K
NH
2
–  NH
3
+
+ H
+

H N – H N
3 3
? ?
2
b
K
Cases

?   Case I : H
2
S (0.1 M
K
a
1
= 10
–4
,
K
a
2
= 10
–9
)
? H
2
S

H
+
+ H S
–
H S
–

H
+
+ S
2–
0.1       0 0 x        x 0
0.1 – x     x + y x – y x – y        x + y y
?

?

? ?

?

?
0.1        x x x          x y
K
a
1
> >
K
a
2
x    > >  y
? ? ?  x
0.1
x
10
2
4 –
5 –
10
9 –
10
x
y  x
?
?
```
Offer running on EduRev: Apply code STAYHOME200 to get INR 200 off on our premium plan EduRev Infinity!

## Chemistry for JEE

223 videos|452 docs|334 tests

,

,

,

,

,

,

,

,

,

,

,

,

,

,

,

,

,

,

,

,

,

;